A subst of interferon (IFN) stimulated genes (ISG) encodes human 15- kDa (ISG15) and bovine 17-kDa (ISG17) ubiquitin homologs that share similar structure and function. ISG15 is induced in several cell lines in response to IFN and in decidualized human stromal cells in response to pregnancy. ISG17, however, appears to be primarily induced in the bovine endometrium in response to conceptus-derived IFN-tau. These ISGs conjugate to intracellular proteins in response to IFN. The fate and identify of proteins conjugated to these ISGs have not been determined. ISG17 is realized by the endometrium from day 15 to 26 of pregnancy in cows and in endometrial cells treated with IFN-tau. Also, ISG15 has immunomodulatory roles, an example of which is induction of release of IFN-gamma. It is propose to use a bovine model to study the intra- and extra-cellular function of these ISGs. Recombinant (r) ISG17 will be generated using a yeast expression system, purified, and evaluated for bioactivity/function using in vitro and in vivo approaches. Proteins that become conjugated to ISG17 in response to IFN-tau and pregnancy will be identified using antibody-based affinity, molecular biology, and hypothesis-based approaches. Finally, it is proposed to determine tissue distribution and cytosolic trafficking of ISG17 and its conjugates in endometrium during pregnancy and in endometrial cells in response to IFN-tau using epifluorescent and confocal microscopy.